Transcriptional regulation in prokaryotic systems is being studied in vitro at the level of E. Coli RNA polymerase - DNA interactions. Physical and biochemical techniques are being used to identify and characterize the step(s) involved in polymerases binding and initiation that are most sensitive to promoter sequence and thereby determine promoter strength. Procedures include abortive initiation and a new fluorescence technique that directly measures polymerase binding to templates containing 4-thio-thymidine. The spectroscopic probe is particularly suited to measuring weak interactions in solution. Experiments are initially being done with restriction fragments carrying strong and weak promoters of T7 phage DNA and with promoters such as lac that require positive control factors. The possibility of using RNA polymerases I and the extrachromosomal ribosomal RNA genes of Tetrahymena to develop a simple in vitro system for study of selective transcription in eukaryotes is also being explored.